One emerging technology for non-volatile memory is magneto-resistive random access memory (MRAM). A common form of MRAM is based on the tunnelling magneto-resistance (TMR) effect, in which each memory cell comprises a magnetic tunnel junction (MTJ). Such an MTJ may be formed from two ferromagnetic metal layers, with an insulating layer placed between the metal layers. When a voltage is applied between the metal layers, a tunnel current flows. The tunnel resistance varies based on the relative directions of magnetization of the metal layers. The tunnel resistance is small when the directions of magnetization are parallel (typically representing a “0”), and large (approximately 10%-20% higher, at room temperature) when the directions of magnetization are anti-parallel (typically representing a “1”).
The metal layers in a typical MRAM MTJ include a “fixed” layer, in which the direction of the magnetization is fixed, and a “free” layer, in which the direction of the magnetization can be switched by application of currents. These currents are typically applied through conductive write lines referred to as bit lines and word lines, which are disposed so that the bit lines are orthogonal to the word lines. In an MRAM array, an MTJ memory cell is located at each intersection of a bit line with a word line.
In a typical MTJ cell, to switch the direction of magnetization of the free layer, of a particular cell, currents are applied through the bit line and the word line that intersect at that cell. The direction of these currents determines the direction in which the magnetization of the free layer will be set. The combined magnitude of the currents through the word and bit lines must be sufficient to generate a magnetic field at their intersection that is strong enough to switch the direction of magnetization of the free layer.
A more recent type of MRAM cell is a spin-injection MRAM. In a spin-injection MRAM, the free layer is not switched via application of a magnetic field generated by the bit lines and word lines. Instead, a write current is applied directly through the MTJ to switch the free layer. The direction of the write current through the MTJ determines whether the MTJ is switched into a “0” state or a “1” state. A select transistor connected in series with the MTJ may be used to select a particular cell for a write operation.
One problem with MRAM devices is that the signals provided by MTJs on reading are small relative to the signals provided by other non-volatile memory technologies. This small signal makes reliable readout at high speeds difficult, and generally requires that the sense amplifier that is used to read MRAM memory use as much of the cell signal as possible. Thus, reduction of signal losses in the sensing path of the memory is of increased importance in MRAM devices.
High-resistive ground connections of memory cells are one common source for signal loss in memory arrays. While providing a low resistive ground connection is relatively straightforward when the memory cells can be permanently tied to ground, it can be difficult when the ground connection has to be switched, as is typically the case in spin-injection MRAM, and in other types of memory that use bi-directional current for writing. Because it must be able to force a write current through the MTJ in either direction, typical spin-injection MRAM designs do not have fixed ground connections. This causes noise and signal loss in the sensing path of spin-injection MRAM devices. Similar problems may be encountered in other memory devices that use bi-directional current for writing.
What is needed in the art is a design for a memory array that uses bi-directional current for writing, and that reduces signal loss in the sensing path due to switched ground connections.